The aluminum nitride has the excellent characteristics such as about twice as larger bandgap (6.2 eV) compared to gallium nitride or silicon carbide which is known as the wide bandgap semiconductor, high dielectric breakdown voltage (12 MV/cm), and high thermal conductivity (3.0 Wcm−1K−1), and it is expected as a material capable of realizing a power device having even higher withstand voltage characteristic than the case of using the above mentioned materials.
As the device structure for realizing high performances (a high capacity and a high withstand voltage), in case of using the device using gallium nitride and silicon carbide, the vertical semiconductor device structure wherein the conductive substrate is used as the supporting substrate, in which the current flows or the voltage is applied to between the surface of the element and the backside, is proposed (see the patent documents 1 to 3).
By employing the vertical structure, the withstand voltage characteristic can be improved which was the object for the device of the horizontal type that is for the device wherein the current flow for driving the device runs in the horizontal direction. Further, by employing the vertical type structure, a larger electric power can be applied to the device. In order to realize the vertical type device structure, as mentioned in the above, it is necessary to use the conductive substrate. In regards with the conductive aluminum nitride, the method of forming the n-type conductive crystal layer doped with Si by a metalorganic vapor phase epitaxy (MOVPE) or a hydride vapor phase epitaxy (HVPE) or so is known (Patent documents 4 and 5).
However, in the patent documents 4 and 5, the n-type aluminum nitride single crystal layer is formed on the foreign substrate such as SiC substrate; hence it was difficult to produce a single crystal layer with high crystalline quality. Particularly, there was a room for improvement to produce the high crystalline quality and thick n-type aluminum nitride single crystal layer.
In order to solve above mentioned problems, the method of growing the n-type aluminum nitride single crystal layer on the base substrate made of the same material that is of the aluminum nitride single crystal layer has been developed (See the patent document 6).    [Patent Article 1] JP Patent Application Laid Open No. 2003-086816    [Patent Article 2] JP Patent Application Laid Open No. 2006-100801    [Patent Article 3] JP Patent Application Laid Open No. 2009-059912    [Patent Article 4] JP Patent Application Laid Open No. 2000-091234    [Patent Article 5] JP Patent Application Laid Open No. 2003-273398    [Patent Article 6] JP Patent No. 5234968
According to the method disclosed in the patent document 6, after the aluminum nitride freestanding substrate having a threading dislocation density of 105 cm−2 or so by the method disclosed in JP Patent Application Laid Open No. 2005-2552248 is produced, the n-type aluminum nitride single crystal layer can be formed on the aluminum nitride freestanding substrate. Then, according to this method, the n-type aluminum nitride single crystal layer having the dislocation density of 5×109 cm−2 or so can be obtained.
However, the method of the patent document 6 has only produced a thin n-type aluminum nitride single crystal layer having the layer thickness of 10 μm or less, and there is no specific example of forming the electrodes. That is, the conventional art has still not realized the vertical semiconductor device with the thickness so that the n-type layer can be used as the freestanding substrate, and using the high quality aluminum nitride single crystal with n-type conductivity.
The present invention is attained in view of such situation, and the object is to provide the vertical semiconductor device using the n-type aluminum nitride single crystal substrate doped with Si.